The problem I had with a buck converter was that it was rated like the one Lefty pointed out with a max input of 35VDC. When you rectify and lightly filter a 24VAC supply, it comes out around 37VDC which is just over the max for the buck converter. In my case I tried two of them, neither one was the one Lefty indicated, and they both got hot and failed in a couple of days.
Lefty may have tried the one he shows at that level and had a good experience, but I had both of mine release small amounts of magic blue smoke. They didn't work too well after that. You might have better luck looking around for one that can handle 40VDC or better input to give you a fighting chance of a reasonable lifetime.